Nurse Shark

Nurse Shark

Ginglymostoma cirratum

These bottom dwelling sharks are usually yellowish-tan to dark brown, and average around 8 to 9 feet long and over 200 pounds. They are nocturnal, scouting the sea bottom for crustaceans, mollusks and stingrays during the night, then returning to their preferred cave or crevice where they will often lay together in groups to sleep during the day. Nurse sharks are not aggressive, but there are attacks on record, often resulting from humans acting incautiously when sharing the water with the sharks.

Common Names

The term "nurse" probably is derived from "Nusse", the common name originally applied to catsharks of the family Scyliorhinidae, to which G.cirratum then was thought to belong. In Caribbean waters, the nurse shark is still often referred as "tiburon gato" or cat-shark.

Importance to Humans

At present there is not a fishery for this species. The fins are not marketed and the meat, although edible, is not often retained for human consumption. However it is sometimes sold as crab bait.

Nevertheless, nurse shark are caught and killed by fishermen in some regions because they are considered a nuisance animal that takes bait intended for other species. In the Lesser Antilles, where it often raids fish traps, it is considered a pest. Commercial fishers in the United States routinely release nurse sharks alive.

In the past nurse sharks were sought for various reasons. The liver oil often was used as fuel. The oil also was used by commercial sponge fishers to calm the water surface, allowing them to more readily locate sponges on the seafloor. The skin was considered the best of all elasmobranch species, being extremely tough and thick, and was used to make high quality leather. The skin also was occasionally salted for human consumption.

Danger to Humans

Mainly non-aggressive, generally will swim away when approached. However, some unprovoked attacks on swimmers and divers have been reported. If disturbed it may bite with a powerful vice-like grip capable of inflicting serious injury. In some instances, jaws release was accomplished only after using surgical instruments. The frequency of bites has increased recently as a result of ecotourism feeding operations.

Conservation

Although the nurse shark is not an endangered species, its abundance in the littoral waters of Florida has decreased in the past decades. The presence of this species in areas with constant anthropogenic activity makes it susceptible to disturbance. Because of its relatively docile behavior, swimming with (and handling and feeding) nurse sharks is a very popular form of ecotourism.

The effects of high levels of interactions with humans in coastal waters are not completely known, but the fact that nursery areas are now limited only to remote regions suggests a correlation.

Identification and protection of these areas, coupled with further research on the biology of nurse shark, is essential to provide effective conservation.

The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.

Geographical Distribution

World distribution map for the nurse shark

Common in the Atlantic and in the eastern Pacific, in coastal tropical and sub-tropical waters. Reported from Senegal to Gabon, Rhode Island to Southern Brazil, and Mexico to Peru. Also, some individuals have been reported in the Gulf of Gascogne in southwest France. This species is locally very common in shallow waters throughout the West Indies, south Florida and the Florida Keys. Apart of the eastern Pacific, the nurse shark is absent from the Indo-Pacific area, where other related groups have successfully evolved.

Habitat

The nurse shark is a nocturnal animal that rests on sandy bottoms or in caves or crevices in rock in shallow waters during the day. They occasionally occur in groups of up to 40 individual, as they lie very close together sometimes even piling upon one another.

Nurse sharks are very active during the night. In addition to swimming near the bottom or well off it, the nurse shark can clamber on the sea floor, using its flexible, muscular pectoral fins as limbs. Large juveniles and adults are usually found around deeper reefs and rocky areas at depths of 3-75 meters (10-250 ft) during the daytime and migrate into shallower waters of less than 20 meters (70 ft) deep after darkness.

Juveniles up to 170 cm (6 ft) are generally found around shallow coral reefs, grass flats or mangrove islands in 1-4 meters (3-13 ft) of water. They often lie in groups within limestone solution holes or under rock ledges.

Nurse sharks show a strong preference for certain resting sites, and repeatedly return to the same caves and crevices after a nocturnal activity.

6. First dorsal fin originates well behind the pectoral fins and over or behind the origin of the pelvic fins

Biology

Distinctive FeaturesNurse sharks have two spineless, rounded dorsal fins with the first dorsal fin much larger than second, and one anal fin. The origin of the first dorsal fin is about over the origin of the pelvic fin. The caudal fin is more than ¼ of the total animal length.

The sub-terminal mouth is placed well in front of the eyes, the spiracles are minute, and moderately long barbels reach the mouth. Nasoral grooves are present, but there is no perinasal groove.

ColorationAdult nurse sharks generally range from light yellowish tan to dark brown in color. Juveniles up to 60 cm (23 in) have small black spots, with an area of lighter pigmentation surrounding each spot, covering the entire body. These are bands of lighter and darker pigmentation along the dorsal surface. Juveniles (70-120 cm / 28-48 in) are capable of limited color changes. In a tank experiment small nurse sharks, covered for just a few minutes became considerably lighter than individuals exposed to full sunlight. Unusually pigmented individuals (e.g. brilliant yellow or milky white) have been reported several times.

DentitionNurse sharks possess independent dentition, the simplest type of tooth arrangement found in sharks. This means that there is no overlap between teeth, and that forward movements of teeth leading to shedding does not depend on other teeth. In sharks with various degree of overlapping dentition, replacement of teeth cannot take place until outer blocking teeth are lost. Replacement rates among juveniles are generally faster than for adults. Also teeth replacing occur faster in summer, when water temperatures are higher.

Food HabitsA nocturnal predator, the nurse shark feeds mainly on fish especially stingrays, molluscs (octopi, squids and clams) and crustaceans. Algae and corals are occasionally founded in the stomachs as well. The nurse shark has small mouth, but its large, bellows-like pharynx allows it to suck in food items at high speed. This system probably allows the species to prey on small fish that are resting at night, species that are too active for the sluggish nurse shark to catch during the day. Heavy-shelled conches are flipped over, and the snail extracted by use of suction and teeth.

Young nurse sharks have been observed resting with their snouts pointed upwards and their bodies supported off the bottom on their pectoral fins. Some suggest this posture may possibly provide a false shelter for crabs and small fishes that the shark ambushes and eats.

ReproductionThe nurse shark is an ovoviviparous species. This means that embryonic development occurs in an egg case within the mother's ovary. The embryo has its own yolk sac, which is absorbed during development, and there is no placental nourishment from the mother. The nurse shark has a biennial reproductive cycle. After mating, gestation takes about six months. The young are born in late spring/early summer with litter of 20-30 pups. Each pup measures 10.6-11.8 inches (27-30 cm) total length. It then takes another eighteen months for the ovaries to produce mature eggs for the next breeding. In the waters of the Florida Keys and the Dry Tortugas archipelagos, the reproductive behavior of the nurse shark has been regularly observed and studied, making it's copulatory behavior among the best known in shark species.

Males approach females that resting on the sea floor or are swimming just above it. The male then bites one of the female's pectoral fins simultaneously pushing, trying to turn her onto one side. In this position it is easier for a male to insert his clasper, vigorously bending the lower portion of his body towards the female's cloaca.

A large number of males generally try to mate with a single female, often leading to females bearing numerous scars and bruises from male bites. Females frequently try to avoid contact with males by swimming in very shallow water, where they can bury their pectoral fins in the sand.

PredatorsThere are no species that regularly prey on nurse sharks. However some larger sharks are known to feed occasionally on them. Remains of nurse sharks have been found in lemon shark and tiger shark stomachs, and attacks on nurse sharks by bull sharks and great hammerhead sharks have been observed.

ParasitesNematodes have been observed in the gills of nurse sharks in captivity at the New York Aquarium, U.S.

Taxonomy

The nurse shark was first described by Bonnaterre (1788), as Squalus cirratus. The current scientific combination,Ginglymostoma cirratum was first assigned by Muller and Henle in 1841. Gynglymostoma is derived from the Greek words "gynglimos" = hinge and "stoma" = mouth. The species name cirratum is translated from Latin as curl. Synonyms used to refer to this species in past scientific literature are Squalus punctulatus Lacepède 1800, Squalus punctatus Bloch & Schneider 1801, Squalus argus Bancroft 1832, Ginglymostoma fulvum Poey 1861, and Ginglymostoma caboverdianus Capello 1867.

Similarities in the reproductive cycle may indicate a close relationship between the nurse shark family Ginglymostomatidae and the whale shark family Rhincodontidae. Members of the order Orectolobiformes probably are most closely related to the mackerel sharks of the Order Lamniformes.